The present invention relates generally to vacuum plasma processor methods and apparatus and more particularly to a vacuum plasma processor method and apparatus wherein a constant, non-zero AC parameter is maintained between an electrode having a connection to ground such that no AC or DC source is DC coupled with the electrode.
Vacuum plasma processors are used to deposit materials on and etch materials from workpieces that are typically semiconductor, dielectric and metal substrates. A gas is introduced into a vacuum plasma processing chamber where the workpiece is located. The chamber pressure is typically in the range of 0.1 to 1000 torr. The gas is ignited into an RF plasma in response to an RF electric or electromagnetic field. The RF field is provided by a reactive impedance element, usually either an electrode array or a coil which couples both magnetic and electrostatic RF fields to the gas. The reactive impedance element is connected to an RF source having an RF frequency and sufficient power such that the gas is ignited into the plasma. Connections between the source and reactive impedance element are usually by way of a relatively long cable, connected directly to the RF source. A resonant matching network connected between the cable and reactive impedance element usually includes at least one variable reactance adjusted to match the impedance of the source to the load it is driving.
The load seen by the source is subject to substantial random, unpredictable variations. The load has a relatively high impedance prior to ignition of the gas into a plasma state. In response to the plasma being ignited, the load impedance drops substantially due to the presence of charge carriers, i. e., electrons and ions, in the excited plasma. The ignited plasma impedance also changes substantially during processing of workpieces due to variations in the plasma flux, i.e., the product of the plasma density and the plasma charge particle velocity.
The RF losses are also influenced by the physical makeup of the RF delivery path of the plasma processor, the hardware of the processor chamber and ground path impedance, which is subject to considerable variation at the RF frequencies exciting the plasma. The ground path impedance is determined by the physical makeup of the parts forming the ground, as well as the associated impedance of the ground path at the RF excitation frequency. The load is subject to these variations during processing of a single workpiece. In addition, the load is subject to these variations while the same processor is processing different workpieces. Further, the load is subject to these variations among different processors having the same nominal design, such as the same model number, because different processors have different unpredictable characteristics. The losses and impedance differences have a large effect on the performance of the processor, for example etch and deposition rates.
Previously, it was thought that control of the matching network variable reactance and the output power of the RF source could provide adequate compensation for these random, unpredictable variations. The matching network variable reactance is controlled to maintain an impedance match and resonance between the RF source output impedance and the load impedance.
In addition, control is frequently provided in response to a voltage probe connected between the matching network and the reactive impedance element. The voltage probe derives a signal indicative of the RF voltage between the reactive impedance element and a reference potential, such as ground, the potential at which a metal wall of the chamber is maintained. The signal indicative of the RF voltage between the reactive impedance element and the reference potential is coupled to a controller for an output parameter of the RF source, which is remote from the chamber and matching network. The controller is usually part of the RF source and includes a monitor for the current and voltage the RF source applies to an end of the cable connected to the RF source. The monitored current and voltage, either from the voltage probe connected between the matching network and reactive impedance element or the voltage monitor of the RF source, are combined to control output power of the RF source to a desired setpoint.
The assumption has been that the current monitored in the controller of the RF source is an accurate replica of the current flowing in the reactive impedance element and load. I have realized that this is an invalid assumption because of the loss effects of the cable connected between the RF source and the matching network, as well as other associated losses. In addition, the foregoing load variations have an adverse effect on this assumption. Because of this invalid assumption, desired power has not actually been supplied in many instances to the plasma during processing, with a resulting adverse effect on processor performance.
In accordance with the present invention, a plasma processor includes a vacuum plasma chamber for processing a workpiece, wherein the chamber includes a reactive impedance element for electrical coupling with gas in the chamber and an electrode having a connection to AC ground such that no AC or DC source is DC coupled with the electrode. The connection of the electrode to AC ground is such that a finite, non-zero AC voltage has a tendency to be developed between the electrode and AC ground. Sufficient power is supplied to the reactive impedance element to excite the gas in the chamber to a plasma while a constant finite, non-zero AC parameter, preferably voltage, is maintained between the electrode and AC ground in the connection.
Preferably the constant voltage is maintained between the electrode and AC ground in the connection by detecting AC voltage between the electrode and ground and/or AC current flowing between the electrode and ground. In response to the detected voltage and/or current in the connection, AC impedance between the electrode and ground in the connection is controlled to provide the substantially constant, finite non-zero AC parameter.
In one preferred embodiment, the AC impedance includes a variable reactance, preferably an inductor and/or resistor, and/or capacitor having a value controlled by the detected voltage and/or current in the connection. The detected voltage and/or current in the connection are used to control (1) whether the inductor, capacitor or resistor, or which combination thereof, is part of the connection and (2) the value(s) of the connected impedance(s).
As a result of the foregoing, ground impedance is maintained at a constant value to assist in accurately controlling power delivered to the plasma. Together, accurate control of delivered power and maintaining ground impedance at a constant value enable a user of the processor to have almost total control over many AC parameters having a large effect on processor performance. This is especially advantageous when matching multiple processors running the same process and maintaining process stability on the same processors over long time periods. The principles of accurately controlling delivered power and maintaining ground impedance at a constant value can also be used to maintain constant characteristics during the manufacture of processor can be accurately measured and adjusted to guarantee performance during manufacturing of the processors, prior to shipping the processors to the end-user.
Plasma processors that maintain a constant finite non-zero AC parameter between the electrode and AC ground in the connection are preferably, but not necessarily, employed with arrangements for controlling an AC source driving a reactive impedance element maintaining constant power across a load including the reactive impedance and the plasma it excites.
The above and still further objects, features and advantages of the present invention will become apparent upon consideration of the following detailed description of a specific embodiment thereof, especially when taken in conjunction with the accompanying drawings.